A tempered glass sheet, one kind of safety glass sheets, has been used extensively as a window glass sheet for buildings, automobiles or the like. As a method for manufacturing such a tempered glass sheet, an air-cooling method has been widely used, which comprises quenching a glass sheet that has been heated to a temperature around its softening point by blowing a cooling air to form a residual compression stress layer at the surface of the glass sheet.
On the other hand, the demand for a glass sheet with a predetermined curved shape has become higher, especially in the field of automobile windows to meet the requirement for automobile design and an aerodynamic property. Therefore, in the air-cooling method, the heated glass sheets are often bent before blowing a cooling air. Some methods are known as a method for bending a glass sheet while tempering. For example, in one method, a glass sheet is heated and pressed by a pair of convex and concave press dies while the glass sheet is suspended by tongs (hoisting accessories). In another method, a glass sheet that has been heated while horizontally carried in a heating furnace is pressed by a pair of upper and lower press dies.
In such bending methods utilizing a pair of press dies, the step of softening a glass sheet by heating and the step of bending the glass sheet are basically separated. However, a method for conducting these steps as the same step has been proposed. In this method, a glass sheet is heated while carried by a glass sheet carrying system such as a roll or a bed having a predetermined curvature in a heating furnace so that the heated glass sheet sags gradually due to its self-weight, finally forming a curved surface having the above curvature. The bent glass sheet is usually quenched in a quenching apparatus located adjacent to the heating furnace. Such a method is excellent because it does not require the step of pressing a glass sheet. In order to reap full advantage of the method, various improvements have been conventionally provided to be put to practical use (Publication of Japanese Examined Patent Application No. 44-14832, Publication of Japanese Examined Patent Application No. 48-5242, and Publication of Japanese Unexamined Patent Application No. 7-237928).
For example, Publication of Japanese Unexamined Patent Application No. 7-237928 discloses a method comprising blowing a gas on one surface of a glass sheet, while the glass sheet is carried out from a heating furnace to a quenching apparatus, to provide a difference in temperature in the temperature range of its softening point or above to intentionally provide a difference in a contraction rate for both surfaces of the glass sheet. This method aims at improving the degree of freedom for forming a glass sheet without limiting the shape of the glass carrying system.
However, the method described in the Publication of Japanese Unexamined Patent Application No. 7-237928 requires further providing a gas blowing system between the heating furnace and the quenching apparatus for a glass sheet. Also, this method requires discharging a gas from the gas blowing system at a pressure basically higher than that of the cooling gas. Therefore, the manufacturing method cannot be conveniently implemented by the conventional apparatus.